Apparatus for melting and casting



Feb. 15, 1966 Filed Sept. 6, 1962 H, R. SMITH, JR 3,234,606 APPARATUSFOR MEL'IING AND CASTING 3 heetsSheet l INVENTOR.

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zwzmw United States Patent 3,234,606 APPARATUS FOR MELTING AND CASTINGHugh R. Smith, Jr., Piedmont, Califi, assignor to TernescalMetallurgical Corporation, Berkeley, Calif., a corporation of CaliforniaFiled Sept. 6, 1962, Ser. No. 221,807 7 Claims. (Cl. 22-73) The presentinvention relates to an improved method and apparatus for pouring moltenmetals and particularly metals or materials having very high meltingpoints.

The melting and casting of materials having high melting points,particularly the refractory metals, such as tungsten, titanium,tantalum, niobium, and molybdenum is difficult in that they areextremely reactive at high temperatures and are very susceptible tocontamination during processing. Vacuum processing of thesehigh-meltingpoint-materials avoids contamination and is advantageousover sintering processes, because, for example, certain residualimpurities, such as hydrogen, oxygen, carbon, nitrogen or other volatileimpurities, with relatively low boiling points, are easily removed. Byelectron bombardment heating these metals to a high temperature in avacuum, impurities are easily evaporated therefrom and readily removedfrom the furnace. In many castings, the temperature gradient in thecasting must be religiously controlled to obtain proper directionalsolidification. In order to accomplish this, additional metal isconventionally poured into the mold to form a large, heated hottop orriser, which transfers heat to the cooler metal in the bottom portion ofthe casting to prevent undesirable stresses that might otherwise developduring the cooling process. This problem is particularly acute incasting the refractory metals. Another serious difi'iculty is castingrefractory metals is that upon termination of the heating process, themetal may solidify in the crucible before the crucible is emptied. Thiscreates a peripheral mass of solid met-a1 about the walls of thecrucible. Such solidification then necessitates additional melting,which is timeconsuming, expensive, and ineflicient. The foregoing isparticularly undesirable in the formation of large castings andadditionally demands a high degree of care in the casting process toremain within critical directional solidification limitations.

The above problems of peripheral solidification and directionalsolidification become accentuated when the ad vantages of vacuumprocessing are desired, since the repeated heating and pouring processesmust be remotely controlled.

The present invention provides an improved method of meltinghigh-melting-point metals, especially in a vacuum, and overcomes theproblems present in this type of casting by continuing the heating ofthe metal in the crucible while the crucible is tilted for pouring theliquid metal into a mold. For this purpose, an electron gun is mountedupon one side and near the top of the crucible or upon some part of thestructure that moves in unison with the crucible, while the crucible istilted for pouring molten metal. When ultra purity is desired, the metalmay be maintained in molten state within a vacuum for any desired periodof time, either in the crucible or in the mold, causing impurities toevaporate into the vacuum system for removal by the vacuum pumps. Thus,the invention provides for melting, purification and casting in onecontinuous and uninterrupted process.

The entire process is completed within a vacuum chamber, so that theknown advantages of vacuum operation are obtained. The electron beam isfocused, by a magnetic field, onto the surface of the metal within thecrucible to heat and melt the metal within the crucible and to maintainat least part of the metal in a molten state. In pouring liquid metal,the crucible is tilted and the electron 3,234,606 Patented Feb. 15, 1966beam continues to bombard the metal, so that some of the metal is alwaysmaintained in a liquid state. In the case of the refractory metals, asolid residue is left about the inner surface of the crucible andprotects the walls from the very high temperatures employed in melting.Since the special relation of the electron beam and the crucible remainsunchanged during pouring, heat will be continuously applied to the solidresidue for melting more metal as needed and providing a continuoussource of liquid metal. This then eliminates the problem of prematuresolidification of substantial amounts of the material, and a largecasting may be poured without interruption to obtain a freedom fromcracking otherwise resulting from pouring liquid metal on top ofsolidified metal. Addi tionally, in the present invention, the need fora large hot top or riser is eliminated because the electron beam may beswitched, periodically, from the metal in the crucible to the top of thecasting in the mold, for supplying heat thereto, by adjusting the fieldstrength or the focusing electromagnetic field. This provides improveddirectional solidification.

Various objects and advantages of the method and apparatus of thepresent invention will become apparent from the following descriptionand from the accompanying drawings illustrating certain preferredembodiments of the apparatus hereof. In the drawings:

FIGURE 1 is a transverse sectional view of a vacuum furnace including apouring crucible in accordance with the present invention;

FIGURE 2 is a transverse sectional view of the furnace showing acrucible in pouring position above a casting mold;

FIGURE 3 is a transverse sectional view showing the crucible in tippedor pouring position with the electron beam focused into the open top ofa casting mold; and

FIGURE 4 is a transverse sectional view of an alternative embodiment ofthe present invention with the crucible in the pouring position andelectron beams focused into both the crucible and casting mold.

The present invention overcomes the problems referred to above byproviding an improved method of casting metals, and, in particular,there is provided an improved method of high temperature casting. Inthis method, heat is applied to the top of the solid stock material toform a molten pool of hot, liquid metal. Heat is then continuouslyapplied to the top of the metal as the liquid is poured from the top forcasting. Alternatively, or simultaneously, heat is applied to the top ofthe poured metal, within a mold below the solid material. As the liquidmetal is poured from the top of the solid metal stock, more heat isapplied, to melt more stock as more liquid is required or to maintainthe metal molten until later needed. A variety of suitable heating meansmay be employed, but the preferred method employs bombardment heatingmeans. For example, an electron beam is directed onto the top of metalstock contained Within a cooled crucible placed above the metal to becast, which, for economic considerations, may be provided as pieces ofscrap metal. The volume about the crucible and mold is evacuated and theelectron beam is focused onto the metal, to heat and melt the metal. Asthe scrap metal melts, it drips through the spaces between the variouschunks of scrap metal, until it reaches the cold crucible walls. There,the liquid metal will solidify and form a solid peripheral coatingagainst the relatively cold walls of the cooled crucible. This processwill hereinafter be referred to as peripheral solidification, and itshould be noted that, without peripheral solidification, moltentungsten, for example, would melt the walls of most crucibles. As moreelectron energy is applied, a molten pool of metal is formed on top ofthe solid metal, and, when desired, the liquid is poured into the moldby tipping the crucible. At this point, peripheral solidification whichwas useful as an insulator and which aided in protecting the cruciblefrom melting, becomes a hindrance, because the solid metal thereof isnot available for casting, and conventionally repeated meltings andpouring are required. However, in this invention, the electron beam iscontinuously focused into the crucible and energy is applied to thesolid peripheral layer of metal to melt the same, as desired. Thus, asmore liquid metal is required, it is melted, or, when the mold is full,the beam is turned off and the melting and casting process terminated.

The same method may be applied to a solid block of material containedwithin a crucible, or, alternatively, a solid block may be heated untila pool is formed in the top thereof, and the remainder of the unmeltedblock serves as a consumable crucible and as a source of more liquidmetal.

As an additional and optional step in the method, the electron beam isalso focused onto the top of the casting, or a second electron beam issimultaneously employed for the same purpose. In either event, heat isdirectly supplied to the casting, through the use of electron energy.Thus, the temperature of the top of the casting is controlled, and, incastings requiring close control of the temperature gradient therein,directional solidification of the casting is facilitated by directlyheating the top of the casting with the electron beam. In this manner,the present invention eliminates the need for a large mass of moltenmetal otherwise required in such castings and usually supplied bypouring an excessive amount of liquid metal to form a hot top or riser,which is later wastefully removed from the finished casting. Thus, lessmetal is required to form the same casting and greater control of thetemperature of this hot top is available by employing an electron beamto heat the same.

A preferred embodiment of the apparatus of the present invention isillustrated in FIGURE 1 and includes a cup-shaped crucible 11 mountedupon a pivot 12, which is in turn carried by a supporting arm 13. Anelectron gun 14 is rigidly mounted on the crucible 11 and all of theseelements are contained within a vacuum chamber 15 defined by structure16 and evacuated by a conventional vacuum pump 17.

The crucible 11 is formed as a container with an open, or partiallyopen, top capable of holding chunks of ore, pieces of scrap metal, orshavings of metal, and also capable of holding molten metal aftermelting. The crucible may have any desired configuration, and, in thepreferred embodiment of FIGURE 1, it is shown as a hollow, cylindricalcontainer closed at the bottom and open at the top. The crucible wallsdefine cooling passages 18 connected to a plurality of outlets 19,through which cooling fluid 20 may be circulated. Flexible tubes (notshown) may be provided for circulating cooling fluid. Cooling means areprovided because most available materials suitable for crucibleconstruction are either reactive with the refractory metals at elevatedtemperatures, as of the order of 3500 F., or will melt for below thistemperature.

The pivot 12 is mounted on the supporting arm 13 (which may be mountedagainst the housing 16, as shown) so that its pivotal axis ishorizontally disposed, allowing crucible 11 to be rotated thereabout inthe vertical plane. The pivot is attached to crucible 11 at anyconvenient point which allows the crucible to be tilted in the verticalplane for pouring out molten material. Crucible 11 is also provided witha tilting mechanism 23, which may, for example, include a hydrauliccylinder 24 with an internal piston 25 connected to a tilting arm 26 onthe crucible 11 through a push rod 27. A pump 28 actuates the piston 25through fluid feeding lines 30 and 31 with suitable valving or controlmeans (not shown) for moving the push rod 27 to the right or left forpivoting the crucible.

Electron guns are well known n the and y C011- ventional type ofelectron gun may be employed herein for bombarding material in thecrucible. For example, the electron gun 14 may be of the type containingan electron emissive cathode 35, an accelerating anode 36, and anelectromagnet 37 establishing a magnet field for bending and focusingthe electron beam 38. This magnetic field has lines of force generallyin a direction perpendicular to the plane of the drawings and thusperpendicular to the direction of beam travel. The field causes the beamto bend, as shown, in accordance with well known electromagnetic theory.The electron source 14 is rigidly mounted on the crucible 11, oppositethe intended pouring side, and the field strength of the electromagnet37 is adjusted to bend the electron beam 38 around and onto the surfaceof a metal 39 contained in the crucible 11. The beam 38 providessufficient energy to melt the stock metal 39, forming a pool 40 of hot,liquid metal atop of a solid residue 41 formed by peripheralsolidification, and which is maintained solid by the relatively coldwalls of the crucible 11. When the crucible 11 is tipped about the pivot12, the electron source 14, being secured to crucible 11, will turn withthe crucible so that the electron beam 38 is maintained at the samerelative angle with respect to the crucible walls and remainspermanently focused on top of the metal 39. The beam then continues toheat the metal as it is poured. It should be noted that the presentstate of the art in the field of electron sources is such thatsatisfactory use of electron beams in the higher pressure ranges hasbeen successfully accomplished. Thus, if the advantages of vacuumpurification are not required, the method and apparatus disclosed hereinmay be employed with pressures of the order of several millimeters ofmercury when an electron beam is used as the heating source.

Operation of the apparatus hereof is initiated by charging the crucible11 with material to be melted and evacuating the chamber 15. Additionalmaterial may be placed in the crucible through a vacuum lock 43,preferably disposed at the top of the housing above the crucible. Withthe crucible filled and in a horizontal position, as shown in FIGURE 1,the electron source is energized to generate an electron beam and tofocus the beam into the crucible and onto material therein.

In the melting and casting of low temperature metal, such as copper,substantially all of the material may be melted, as the temperaturesemployed need not be sufficiently high to damage the crucible. With hightemperature metals, the above-noted peripheral solidification providesan insulating layer of metal about the interior of the crucible.Electron beam energy is controlled to provide a desired amount of heatto the material for maintaining a molten pool thereof within thecrucible, and the cooling fluid, which is passed through the cruciblewalls, maintains the crucible temperatures within a safe range. Forexample, the crucible may be formed of copper (melting point 1981 F.)and refractory metals (having melting points above 3000 F.) operatedupon therein without crucible damage.

Following initial melting of the material within the crucible to form amolten pool 40, the crucible is pivoted into the position shown inFIGURE 2, so that molten metal flows from the crucible downward into themold 44. During and following this pouring of material from thecrucible, the electron beam 38 remains focused within the crucible sothat material therein is maintained liquid to facilitate pouring. Themold 44 is also provided with passageways 45 and a plurality of outlets46, through which cooling fluids may be passed to cool mold 44 and toprovide the desired temperature gradient necessary to establish anyrequired directional solidification.

Referring to FIGURE 2, it can be seen that, after the molten metal 40has been poured into the mold 44 by tipping the crucible 11 in thevertical plane about the horizontally-disposed axis of the pivot 12, thefrozen residue 41 will remain in the crucible. Since the electron source38 is securely attached to the crucible 11, it will also be tipped andwill remain focused within crucible 11 upon the frozen residue 41 formelting the same and providing a continuous supply of liquid metal 40,even in the tipped position. In order to compensate for any change inlevel of the molten pool 40 within the crucible because of tipping, theelectron beam is adjusted so that it remains on any part of the surfaceof the pool 41? contained in crucible 11. Should no more liquid bedesired, the electron beam 38 is extinguished by cutting off the supplyof current to filament 35, thereby stopping the supply of heat to pool40, causing pool 40 to solidify. On the other hand, if a greater amountof liquid be desired, the beam may be intensified, accelerating themelting of the solid residue 41 and providing more liquid without thenecessity of restoring the crucible to its upright position forreloading or reheating. In this manner, a continuous source of liquidmetal is provided and liquid may be poured off when desired, therebyobviating the problem of peripheral solidification.

In FIGURE 3 there is shown an alternative electron beam focusing whereinthe beam 38 is directed onto the cast metal 47 which has been pouredinto the mold 44. As the liquid metal is poured, the beam may beswitched back and forth from the frozen residue 41 in the crucible 11 tothe cast metal 47 in the mold 44 as needed simply by switching themagnet energization or the accelerating voltage to vary the beamtrajectory. In this manner, a desired amount of heat is supplied to thematerial in the mold so as to achieve the requisite directionalsolidification.

An alternative embodiment of the apparatus, as shown in FIGURE 4,provides a plurality of electron guns mounted upon the crucible. In thiscase, the electron gun 51 emits an electron beam 52, which is focused onthe metal 53 in the crucible 54, so as to provide a continuous heatsupply thereto. Another electron source 55 emits an electron beam 56,which is focused on the top of the cast metal 57, or the riser thereofin the mold 58, for providing thereat a continuous source of heat.

In either manner, a hot top may be maintained on top of the castingwhich will facilitate directional solidification of the metal in themold. Since many molds must be designed to allow for directionalsolidification of the casting therein, the design problem is hereinsimplified because the added heat applied at the top of the castingalleviates the need for relying solely on heat absorbed from a largeriser provided at the top of the casting to supply heat to the castingas it cools and to control its cooling rate. Because electron energy isused to supply heat, the size of the riser in the mold may be reduced,providing a substantial decrease in the amount of costly materialrequired in the riser, especially when the expensive refractory metalsare cast. This greatly reduces the expense and time in forming castings,and provides greater temperature control.

It should be noted that after melting the metal, the energy of theelectron beam may be lowered so as to just maintain the molten metal inliquid state, allowing evaporation of impurities into the vacuum system.Thus, full advantage of vacuum casting and purification may be attained,and, if ultra purity is desired, as, for example, in the use ofnickel-based, iron-based or cobalt-based super alloys, suitable for usein turbines, the present invention provides a faster and improved methodof continuously casting high-melting-point metals in a vacuum where theimpurities are driven off. Thus, this invention provides a method andapparatus whereby metals are maintained in a molten state throughout theprocess, without the disadvantages presented by undesirable peripheralsolidification.

It should be understood that this invention in its broader aspects isnot limited to specific examples illustrated and described herein, andthat the following claims are intended to cover all changes andmodifications that do not depart from the true spirit and scope of thisinvention.

What is claimed is:

1. An apparatus for melting and casting materials in a vacuum, saidapparatus comprising, an open-topped crucible defining a cavity adaptedto contain materials for melting and movable between a materialretaining and a material pouring position, means defining an enclosedchamber surrounding said crucible, means adapted to evacuate saidenclosed chamber, electron gun means rigidly secured to said crucibleand movable therewith between the material retaining and the materialpouring positions adapted to emit an electron beam capable of meltingsaid material, and means for focusing the electron beam upon the surfaceof the material contained in said cavity while said crucible is in saidmaterial retaining position and upon the surface of non-molten materialremaining in said cavity While said crucible is in said material pouringposition.

2. An apparatus for melting and casting high temperature materials in avacuum, said apparatus comprising, a crucible including means definingan open-topped cavity therein adapted to contain materials for melting,said crucible being movable between a material retaining and a materialpouring position and being provided with cooling jackets adapted todissipate heat therefrom to prevent melting of the material in saidcrucible adjacent said cavity defining means, means defining an enclosedchamber surrounding said crucible, means adapted to evacuate saidenclosed chamber, electron gun means rigidly secured to said crucibleand movable therewith between the material retaining and the materialpouring positions adapted to emit an electron beam capable of meltingsaid material, and means for focusing the electron beam upon the surfaceof the material contained in said cavity while said crucible is in saidmaterial retaining position and upon the surface of non-molten materialremaining in said cavity while said crucible is in said material pouringposition.

3. An apparatus for melting and casting materials in a vacuum, saidapparatus comprising, an open-topped crucible adapted to containmaterials for melting and movable between a material retaining and amaterial pouring position, means defining an enclosed chambersurrounding said crucible, means adapted to evacuate said enclosedchamber, an open-topped mold located within said enclosed chamber andpositioned below the crucible so as to accept the material pouredtherefrom when said crucible is in said material pouring position,electron gun means rigidly secured to said crucible and movabletherewith between the material retaining and the material pouringpositions adapted to emit an electron beam capable -of melting saidmaterial, focusing means for focusing the electron beam upon the surfaceof the material exposed at the open top of said crucible while saidcrucible is in said material retaining position, said focusing meansfurther being adapted to selectively focus said electron beam while saidcrucible is in the material pouring position alternately upon thesurface of the material exposed at the open top of said crucible andupon the surface of the material accepted by said mold and exposed atthe open top thereof to maintain at least a portion of the material insaid mold in a molten state.

4. An apparatus for melting and casting high temperature materials in avacuum, said apparatus comprising, a crucible including means definingan open-topped cavity therein adapted to contain materials for melting,said crucible being movable between a material retaining and a materialpouring position and being provided with cooling jackets adapted todissipate heat therefrom to prevent melting of the material in saidcrucible adjacent said cavity defining means, means defining an enclosedchamber surrounding said crucible, means adapted to evacuate saidenclosed chamber, an open-topped mold located within said enclosedchamber and positioned below said 7 crucible so as to accept thematerial poured therefrom when said crucible is in the material pouringposition, electron gun means rigidly secured to said crucible andmovable therewith between the material retaining and the materialpouring positions adapted to emit an electron beam capable of meltingsaid material, focusing means for focusing the electron beam upon thesurface of the material exposed at the open top of said crucible whilesaid crucible is in said material retaining position, said focusingmeans further being adapted to selectively focus said electron beamwhile said crucible is in the material pouring position alternately uponthe surface of the material exposed at the open top of said crucible andupon the surface of the material accepted by said mold and exposed atthe open top thereof.

5. An apparatus for melting and casting materials in a vacuum, saidapparatus comprising, means defining an enclosed chamber, means adaptedto evacuate said enclosed chamber, an open-topped crucible defining acavity adapted to contain materials for melting and movable between amaterial retaining and a material pouring position pivotally supportedwithin said chamber in a manner so as to cause said crucible to normallyassume said material pouring position, means operatively connected tosaid crucible adapted to releasably support said crucible in saidmaterial retaining position, said last-mentioned means being operable tocause said crucible to assume said normal position to accomplish pouringof said material, electron gun means rigidly secured to said crucibleand movable therewith between said material retaining and materialpouring positions adapted to emit an electron beam capable of meltingsaid materials, and means for focusing the electron beam upon thesurface of the material contained in said cavity while said crucible isin said material retaining position and upon the surface of non-moltenmaterial remaining in said cavity while said crucible is in saidmaterial pouring position.

6. An apparatus for melting and casting materials in a vacuum, saidapparatus comprising, means defining an enclosed chamber, means adaptedto evacuate said enclosed chamber, an open-topped crucible defining acavity adapted to contain materials for melting and movable between amaterial retaining and a material pouring position pivotally supportedwithin said chamber in a manner so as to cause said crucible to normallyassume said material pouring position, a hydraulic cylinder positionedexternally of said chamber, said cylinder including a hydraulic pistonselectively movable therein in response to hydraulic pressure appliedthereto, and a piston rod connected to said piston and movable therewithextending into said enclosed chamber, said rod being operativelyconnected to said crucible whereby movement of said piston in saidcylinder causes movement of said crucible between said materialretaining and material pouring positions, electron gun means rigidlysecured to said crucible and movable therewith between the materialretaining and material pouring positions adapted to emit an electronbeam capable of melting said material, and means for focusing theelectron beam upon the surface of the material contained in said cavitywhile said crucible is in said material retaining position and upon thesurface of nonmolten material remaining in said cavity while saidcrucible is in said material pouring position.

7. An apparatus for melting and casting materials in a. vacuum, saidapparatus comprising, an open-topped crucible adapted to containmaterials for melting and movable between a material retaining and amaterial pouring position, means defining an enclosed chambersurrounding said crucible, means adapted to evacuate said enclosedchamber, an open-topped mold located within said enclosed chamber andpositioned below said crucible so as to accept the material pouredtherefrom when said crucible is in the material pouring position, pluralelectron gun means rigidly secured to said crucible and movabletherewith between the material retaining and the material pouringpositions adapted to emit electron beams capable of melting saidmaterial, focusing means for focusing one of said electron beams uponthe surface of the material exposed at the open top of said cruciblewhile said crucible is in both said material retaining and materialpouring positions, and focusing means for focusing another of saidelectron beams while said crucible is in said material pouring positionupon the surface of the material accepted by said mold and exposed atthe open top thereof.

References Cited by the Examiner UNITED STATES PATENTS 2,937,217 5/1960Baker et al Iii-31 3,068,309 12/1962 Hanks 2273 XR 3,157,922 11/1964Gruber 2273 J. SPENCER OVERHOLSER, Primary Examiner.

MICHAEL V. BRINDISI, WILLIAM J. STEPHENSON,

Examiners.

1. AN APPARATUS FOR MELTING AND CASTING MATERIALS IN A VACUUM, SAIDAPPARATUS COMPRISING, AN OPEN-TOPPED CRUCIBLE DEFINING A CAVITY ADAPTEDTO CONTAIN MATERIALS FOR MELTING AND MOVABLE BETWEEN A MATERIALRETAINING AND A MATERIAL POURING POSITION, MEANS DEFINING AN ENCLOSEDCHAMBER SURROUNDING SAID CRUCIBLE, MEANS ADAPTED TO EVACUATE SAIDENCLOSED CHAMBER, ELECTRON GUN MEANS RIGIDLY SECURED TO SAID CRUCIBLEAND MOVABLE THEREWITH BETWEEN THE MATERIAL RETAINING AND THE MATERIALPOURING POSITIONS ADAPTED TO EMIT AND ELECTRON BEAM CAPABLE OF MELTINGSAID MATERIAL, AND MEANS FOR FOCUSING THE ELECTRON BEAM UPON THE SURFACEOF THE MATERIAL CONTAINED IN SAID CAVITY WHILE SAID CRUCIBLE IS IN SAIDMATERIAL RETAINING POSITION AND UPON THE SURFACE OF NON-MOLTEN MATERIALREMAINING IN SAID CAVITY WHILE SAID CRUCIBLE IS IN SAID MATERIAL POURINGPOSITION.